`Don't confuse internal and external representations of data`

Don't represent an empty box was "?" just because you want it to print that way
One reason why not: It makes it hard to change the output!
For example, to change "?" to " "
In the original version of the program, much unrelated code must change:

    63     $squares = [
    64                 ['?', '?', '?'],
           ...

   214     foreach $x(0..2) {
   215        if ($squares->[0][$x] eq "?") {
           ...

In the modified version, only this one line needs to change:

     for my $row (0..2) {
       for my $col (0..2) {
          print ("<td align=center>"
                . ($squares->[$row][$col] || "&nbsp;") 
                . "</td>\n");
       }
       print ("</tr><tr valign=middle>\n") unless $row == 2;
     }